On 6/21/06, Phillip J. Eby <[EMAIL PROTECTED]> wrote: > At 01:16 PM 6/21/2006 -0700, Guido van Rossum wrote: > >Yeah, but if you have names for your constants it would be a shame if > >you couldn't use them because they happen to be defined in the same > >scope. > > Maybe the real answer is to have a "const" declaration, not necessarily the > way that Fredrik suggested, but a way to pre-declare constants e.g.: > > const FOO = 27
The problem with this is that I don't see how to export this property from one module to another. If module A has the above statement and module B does "from A import FOO" then how does the parser know that FOO is a constant when it's parsing B? (Remeber the parser only sees one module at a time; I don't want to drop this separate compilation facility.) It seems you would still end up with lots of duplicate const declarations (e.g. "from A import const FOO"). And it should also be possible to say "import A" and then use "A.FOO" as a constant. I really don't think this ad-hoc solution is going to work. > And then require case expressions to be either literals or constants. The > constants need not be computable at compile time, just runtime. If a > constant is defined using a foldable expression (e.g. FOO = 27 + 43), then > the compiler can always optimize it down to a code level > constant. Otherwise, it can just put constants into cells that the > functions use as part of their closure. (For that matter, the switch > statement jump tables, if any, can be put in a cell too.) > > >I don't like "first use" because it seems to invite tricks. > > Okay, then I think we need a way to declare a global as being constant. It > seems like all the big problems with switch/case basically amount to us > trying to wiggle around the need to explicitly declare constants. And I don't believe declaring constants is going to work; not without a much bigger change to the language and the way we think about it. This is because "const-ness" isn't a property that you can encode as a new type of object. It is a compile-time property of *names*. The only similar thing in Python is globals. But global declarations are intentionally a bit clunky because we believe overuse of the feature would be a mistake. But we wouldn't want to discourage constant declaration if we had them, so having to repeat the 'constant' keyword in every module that uses a particular constant would be a painful wart. Is your objection purely based on the problems of getting switch to behave the same way inside and outside a function? I'd rather forbid or cripple switch outside functions than either add constant declarations or switch to first-use semantics. -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
